Numerical control is more than twenty years old. It started with some of the early developments by John T. Parsons, at M.I.T., at Bendix and other places, namely in U.S. Pat. Nos. 2,537,427; 3,069,608; 3,226,667 and 3,508,251. The early machines were numerical contouring control machines, such as U.S. Pat. No. 3,226,649 or numercial positioning control (i.e. point-to-point), such as U.S. Pat. Nos. 3,248,622 or 3,291,970.
Later CNC was developed both automatically and manually, i.e. with an override, e.g., U.S. Pat. Nos. 3,828,318, 3,816,723 and 3,854,353. The first of these could be programmed by the machine tool operator without use of codes working directly from the parts drawing.
Thus we can see that the NC and CNC systems have come a complete circle. Starting with magnetic taping of a "dry run" which was then repeated, they have gone to elaborate post-processors which prepared the tape. Now more functions are incorporated into a control system itself which was a minicomputer and is now a microprocessor with all of the functions being done on the machine, although a simulated microprocessor may be used for tape generation. Contouring requires more complex computer tapes but this machine is for point-to-point positioning which can be generated on the machine.
One intermediate step in this development for small shops with short part runs was in the development of machinist programmed CNC with the requirement that these were programmed with thumb wheels which is a known technique for conversion of units from English to digital units. This system is a Manual Data Input (MDI) control system and allows 96 X-Y positions with thumb-wheeled dialing of the dimensional coordinates into memory. Examples would be the Anilam machine, Anilam Electronics Corporation, Miami, Fla. This allows editing or changes to be made prior to machining the first part. Another type is shown in U.S. Pat. No. 3,828,318.
Actually these machines do have some or all of the following features, e.g. 100 memory positions for X and Y coordinates, machine controlled parts programming and editing on the machine via thumb wheels, digital read-out to monitor and display of machine location. Operation can be automatic, semiautomatic and manual, allowing for offsets. Single steps in memory can be changed.
Another development that has taken place is exemplified in U.S. Pat. No. 3,854,353 where the controller has a mass memory having at least two parts, one of which is a random access memory from which machine instructions and tool displacement path data are stored. In addition, there is an arithmetic and memory unit. Thus tool displacement, path definition and machine functions from the punched tape are stored in memory and when these instructions are complete, control is returned to the tape.
Another development was DNC, i.e., direct numerical control. This was developed in 1970 but fell short of the mark because the large computer available in 1965 to 1970 could not supply the data pulses at the rate required for three-axis simultaneous contouring controls. Frequently their computing power was too slow to calculate and control the milling heads to interpolate either linearly, circularly, or parabolically in the time required, i.e. microseconds.
The trend is toward further miniaturization of the individual control in a microprocessor controlled CNC. The advantage is that the user is not required to program the N/C; rather the computational software is stored in PROM (Programmable read-only memory).
The drives for N/C machine tools were possible because of high accuracy of recirculating ball feed screws and the use of silicon controlled rectifier (SCR), servo drives or stepping motor. This invention uses the latter system.
Another measuring system used is precision glass scales which eliminates reliance on lead screws for positioning accuracy.
In the 1970's, the advance shifted to computer numerical control (CNC in which a self-contained N/C system for each machine includes a dedicated computer processor using stored instruction inputted usually by punched tape to perform some or all of the basic numerical functions. Another system was DNC, i.e. direct numerical control which refers to a system of multiple machine tools directly controlled by a central computer. See IEEE Spectrum, March 1977, p. 81-83. In these N/C systems, 90 percent of the trouble occurred in the paper or Mylar tape reader. This invention avoids this problem completely by using magnetic tape for input and storage.
With the trend from N/C to CNC the system changed from hardware to software systems bringing flexibility reduction in hardware and simpler diagnostics. Concomitant with this development was a reduction in the inaccuracies in manufacture, some of which had been brought on by paper or Mylar tape readers generally used in N/C systems. These data programs were generally prepared off-line with special programmers and equipment and they necessitated remaking the program for errors or other reasons. Offsets were also limited.
Computer numerical control allowed improvement in means for correcting errors and possibility of debugging the edited part-data tape. Here the program is placed from the tape reader into core memory (store) where it is read many times. Editing can be done to correct mistakes in cutting speed, cutting tool off size, etc. Some of these CNCs would then cut a new tape after the production run had been made.
Many of these CNC systems had cathode ray tube display for instructions and plotting--all associated with the edit function. Aside from the cost, CNC system complexes are really not intended for on the machine programming by the machinist. This is particularly important in the small shop where runs are short and tapes are being changed often.
Another trouble with some of the early CNC systems was in the speed of the microprocessor which limited required speed for certain cutter compensation and circular interpolation computations. As a result, these had wired logic in these early models, i.e. G.E. Mark Century 1050.